1. Technical Field of the Invention
The present invention relates to optical sensors used in distance measurements, especially those which detect pressure by optically measuring the amount of deformation of a diaphragm.
2. Background Art
Conventionally, many types of sensors (measuring devices) which measure pressure, displacement, distance, or the like are known.
While conventional electrical pressure sensors may be found in a variety of forms, strain gauges for example, these electrical formats have the following disadvantages: (1) Being of electrical form, they require electrical wiring to transmit signals. As a result, they are often affected by electromagnetic interference which can lead to measurement errors. If parts are added to prevent such interference, then these can add to the weight or complicate the circuit structure. (2) They also require at least two signal wires to send and receive electrical signals. Moreover, electrical wiring is much heavier per unit length in comparison to optical fiber, so that the weight of the signal transmission route is greater.
Additionally, many types of optical displacement measurement sensors using interferometers or the like are known. As optical range finders, those which use triangulation methods are widely known.
Conventionally, optical fiber pressure sensors have been developed in order to resolve the problems of the electrical pressure sensors mentioned above. One such example is microbent pressure sensors, which have the following disadvantages: (1) They require a pair of sawtoothed microbending portions (known as microbenders or transducers) in order to microbend the optical fiber, resulting in problems in that (a) the overall size and weight of the sensor are increased; (b) there is a problem in durability due to biting and wear between the microbenders and the optical fiber, so that there is a significant risk of increases in measurement errors; (c) environmental temperature changes can cause the microbender portion to thermally expand, changing the microbending force on the optical fiber and resulting in temperature errors for which compensation is not easy; (d) due to problems such as those mentioned in (b) and (c) above, a high-precision pressure measurement is not possible, with the precision being in the range of .+-.1%. (2) While there is a method wherein optical intensity ratios are measured by using a delay line in order to reduce the influence of fluctuations in the optical intensity of the light source and the transmission route, the overall size and weight of the sensor are increased due to the addition of a delay line to the transducer.
The interferometer-type displacement measurement sensors mentioned above have the following disadvantages: (1) While they are highly precise, they are normally only able to measure up to a range of .lambda./2 (wherein .lambda. is the optical wavelength, for example when .lambda.=850 nm, .lambda./2=425 nm), so that a complicated calculation is required when measuring displacements of more than .lambda./2. (2) Even if the measurement of displacements larger than .lambda./2 as explained in (1) is made possible, extremely high-level and complicated calculations are necessary to measure distances, making them quite costly. Therefore, they are not for general industrial use. (3) Since the device described in (1) is basically a displacement gauge, the amount of displacement of the measured object is not known while the power source to the calculation system is turned off, and only the relative displacement after reactivating the power source is able to be known. Additionally, the devices of (2) also often have configurations which result in the same effect when the power source is turned off.
The above-mentioned conventional optical distance finders using triangulation methods have the following disadvantages: (1) They are not very precise (while differing depending on the range format, usually on the order of .+-.10.sup.-1 mm). (2) They are bulky and heavy due to their complicated optical systems.